What causes an emission nebula to glow brightly, as opposed to a reflection nebula?

Answer

The gas within the cloud being energized by nearby hot, young stars.

An emission nebula forms when the gas within the cloud is energized, causing it to glow brightly, typically due to radiation from nearby hot, young stars.

What causes an emission nebula to glow brightly, as opposed to a reflection nebula?
astronomynebulastar formationmass